The present invention relates to directional stability of motor vehicles, and particularly to a method for modifying the resilient attachment of radius arms and strut rods of motor vehicle suspension assemblies to the frames or bodies of such motor vehicles.
Motor vehicle front suspension assemblies of several general types include either radius arms or strut rods which extend in a generally longitudinal direction or diagonally inwardly from a vertically movable portion such as a lower control arm or an axle on each side of the vehicle to an attachment point on the frame or body of the vehicle. Such radius arms or strut rods are intended to limit the amount of motion of the movable portion of the suspension assembly in a direction generally longitudinal of the vehicle. This is required in order to prevent the steerable wheel on one side from being deflected rearwardly relative to the body of the vehicle, thus swinging both wheels toward that side, when a steerable wheel on that side of the vehicle encounters an obstruction such as a stone or a pothole which momentarily obstructs forward movement of the wheel.
While a radius arm or strut rod, hereinafter called a radius member, might be attached to the frame of a vehicle by a universal joint or a ball joint, such a connection would be expensive to manufacture and would transmit, undamped, the shocks and impulses caused by unevenness encountered in the roadway or off-road terrain being traversed by the vehicle. Additionally, some freedom of motion of the radius member is usually required in a direction longitudinal of the vehicle because of other limitations on the motion of the portion of the suspension to which the radius member is fixedly attached.
In order to provide the desired freedom of motion of the end of a radius member which is attached to the frame or body of a vehicle, it has become customary to mount the end of the radius arm or strut rod in a flexible resilient bushing held by a mounting bracket attached fixedly to the frame or body of the vehicle. Such a resilient connection also isolates the frame and body of the vehicle from the shocks which would otherwise be transmitted through the radius member, and which would then cause an undesirable amount of vibration and resulting fatigue of the occupants of the vehicle.
For operation of a motor vehicle on rough terrain, however, the bushings ordinarily used for attaching a radius arm or strut rod to the vehicle frame or body are frequently too elastic.
The resilient bushings normally provided make it possible for a bump or pothole to move a wheel encountering such an obstacle rearward, together with the control arms or axle supporting the wheel, causing the vehicle to steer toward the side on which such an obstacle is encountered. This condition requires an opposing steering correction by the vehicle's driver, and is at least tiring, if not dangerous, when operating such a vehicle on rough surfaces such as poorly maintained roads or unpaved terrain.
When the bushings attaching a radius member to a vehicle frame become deteriorated they no longer restore the strut rod or radius arm to the correct position after obstacles have been encountered, and eventually the need to realign the suspension assemblies results. Furthermore, such deterioration exaggerates the tendency of many vehicles to be susceptible to unevenness of the road surface.
In some cases deteriorated or undesirably soft or loose radius member bushings can be improved by adjusting a nut used to hold the bushings on a radius arm or strut rod to achieve the desired tightness. For example, Eirhart, Jr., U.S. Pat. No. 3,333,653 and Burkitt, U.S. Pat. No. 3,139,275 teach the use of tightenable nuts to increase the amount of compression on bushings used in this portion of a vehicle suspension assembly to achieve the desired result.
In many motor vehicle suspension assemblies, however, bushings are not adjustably mounted. Furthermore, a metal or other hard spacer cannot be used in conjunction with the ordinary bushings in such a location, because it would be likely to cause unacceptable damage to the soft material of which such bushings are made.
If tightening the bushings is not possible, then the previously available way to improve such a situation is to replace a deteriorated bushing, or to replace original equipment bushings which are too soft with ones of less compressible material, in order to provide a stiffer connection between a radius arm and the vehicle's frame. Either of these procedures is quite expensive because of the amount of labor involved.
As shown in Shimizu et al., U.S. Pat. No. 4,252,339, it is known to control the spring constant of a bushing used for such attachment of a radius arm or strut rod by constructing the bushing of rubber including various numbers of layers of canvas fabric within the bushing in order to limit the elasticity and compressibility of the bushing, while providing the desired amount of freedom of movement of the radius arm or strut rod in its mounting on the frame of the vehicle. Shimizu, et al., however, does not teach how to correct the condition in which a vehicle is originally equipped with bushings which are too compressible, without substantial disassembly of the suspension assemblies and replacement of the bushings with those of the type taught in the Shimizu et al. patent.
Normally, replacement of radius member bushings requires disassembly of a large portion of the involved suspension assembly in order to permit the radius arm or strut rod to be withdrawn from its mounting bracket fixedly located on the vehicle's frame. Thus, while the Shimizu, et al. patent teaches installation of bushings which might improve the condition in which steering stability of a vehicle is excessively affected by rough road surfaces, it does not teach how to accomplish this improvement without undesirably high labor costs.
Additionally, replacement of an originally all-rubber bushing with one made of a harder material such as a polytetrafluoroethylene plastic, as has previously been done, is likely to result in too rigid a connection.
What is needed, therefore, is a method and device for correcting or improving the ability of a vehicle to remain directionally stable despite operation on rough surfaces, without the expense of suspension system disassembly and replacement of the bushings used to mount an end of a radius arm or strut rod to the frame of a vehicle.